Underground mines must be properly ventilated so as to provide a substantially continuous flow of fresh air of sufficient volume to dilute and remove dangerous particulates like rock and coal dust and toxic gases such as CH4, CO, CO2, NOx, and SO2. These gases are created by the combustion of fuel by engines used underground in various applications and from blasting with explosives. Toxic gases can also be released from the strata itself. Methane, CH4, is of particular interest in coal mining since the gas is often found alongside coal deposits and because accumulations of this gas are odorless and can result in underground explosions.
Ventilation also plays an important role in the spontaneous heating of coal in an underground coal mine. If the ventilation rate is too high, heat is carried away by convection. If the ventilation rate is too low, the reaction rate becomes oxygen-limited. It has been found that there is an optimum ventilation flow to produce the maximum rate of temperature rise at the critical ambient temperature. Ventilation controls that are well constructed will reduce air contamination, power and fan maintenance costs.
The basic principle underlying mine ventilation is that air always moves from high pressure regions to low pressure regions. Therefore, in order to get the air to flow from the intake to the exhaust, the exhaust air must be at a lower pressure than the intake. As fresh air is pulled into the mine, contaminated air must be drawn out. The fresh air and contaminated air streams must be segregated to prevent contamination of the fresh air entering the mine and to ensure that fresh air is maintained at a higher pressure than the pressure at the entrance of the exhaust system where contaminated air and fresh air commingle.
If shafts are used as the two main airways, the intake airway is called the downcast shaft, and the exhaust airway is referred to as the upcast shaft. Sometimes one shaft can be split to provide both an intake and exhaust airway. If this pressure difference exists naturally between the two airways, then the mine has natural ventilation. Natural ventilation is one of the two methods of ventilating a mine. The other method is mechanical ventilation where fans are used to create the pressure differential.
Stoppings are used to prevent contamination of intake air with return air and to direct air to where it is needed so as to keep intake air from short-circuiting to the exhaust before it reaches the working area. Seals are also used to contain water or liquid-like mine wastes (tailings or slurry). Failure of a seal or stopping could result in a disastrous inundation of an underground mine or expose miners to unacceptable risks through the contamination of fresh air with toxic gases.
Seals are typically built of concrete blocks, sand fill, or other incombustible material. They are sealed tightly against the floor and ribs (i.e. sides) of a mine roadway so that no air can leak through. Porous stoppings such as concrete block stoppings are usually plastered with a cementitious coating on the high-pressure side to reduce air leakage.
Sometimes stoppings have a door, e.g. air-lock, in them to allow miners to pass through. Man doors are not meant to be ventilation controls, but if a man door is propped open it can affect airflow and may cause intake air to short circuit into the return air.
Because intake and return air frequently cross paths at intersections within the mine, overcasts and undercasts are used to permit the two air currents to cross without the intake air short-circuiting into the exhaust. Overcasts are like enclosed bridges built above the normal back level of the mine. Undercasts are like tunnels built below the normal floor of the mine. Undercasts are seldom used in a mine because they are apt to fill with water or debris which would severely slow down the flow of air through them. Overcasts are used more often and are typically constructed with planar concrete block walls sealed against the ribs and floor, and with some type of airtight roof made of pre-stressed concrete, railroad ties, metal sheeting or steel beams. Steel and other metals can sometimes be difficult to use underground due to fumes caused from welding causing air contamination.
In areas of heavy traffic, such as along long haulage roads, mine doors are usually hung in pairs while being used as ventilation controls. They are used to completely close off a mine passage yet open to allow equipment and people to pass through. Mine doors are generally used to keep air from flowing to areas where it is not needed. Mine doors can also be used to isolate separate splits of air. Mine doors are usually hung in pairs, forming an air lock that prevents unnecessary air flow when one of the pairs is opened. The doors should always be opened and closed one at a time in order to maintain the air lock. Mine doors are always hung so that the ventilating air pressure will push them closed if they are accidentally left open. However, the doors should always be closed after you pass through them. Some doors must be closed manually while others can be closed automatically.
Some mines also use fire doors to control airflow in the event of fire. They are usually built at shaft stations and other strategic locations so that if there is a fire they can be closed to serve as a barrier to the fire and contaminated air. In some mines the fire doors will close automatically when the carbon monoxide in the area reaches a certain level. Some mines will also have rollup doors in shop areas which close automatically when a mine fire warning is given.
When ventilation controls such as seals, stoppings, overcasts, and undercasts are installed in underground mines, they are required to meet the safety standards specified in 30 CFR 75.333. This safety standard requires that ventilation which includes overcasts, undercasts, shaft partitions, seals, stoppings, and regulators be constructed of noncombustible material. Noncombustible material is defined in 30 CFR 75.301 as a material which when used to construct a ventilation control results in a control that will continue to serve its intended function for one hour when subjected to a fire test incorporating an American Society for Testing and Materials, International, ASTM E-119-88 temperature/time heat input, or equivalent. Additionally, the ventilation control must meet a flexural strength that is equal to or greater than a conventional 20 cm hollow core concrete block stopping. The 20 cm hollow core concrete block with mortared joints has been tested and shown to have a minimum strength of 190 kg/m2. ASTM E-72-80 is used to determine the flexural strength. Also, sealants or coatings applied to ventilation controls to reduce air leakage must have a flame spread index of 25 or less. The flame spread index test specified in 30 CFR 75.333 is detailed in ASTM E-162-87. The aforementioned codes, regulations, and specifications are intended to serve as examples only with it being understood that other codes, regulations, and specifications can apply depending on the application and the jurisdiction.
The basis for the safety standard of fire endurance and flexural strength relates to concrete block. Concrete block has long been the material of choice for the construction of stoppings and seals. However, the construction of a concrete block stopping is labor intensive and time consuming. Concrete blocks are heavy, a typical 20 cm wide by 20 cm high by 41 cm long hollow concrete block has an average mass of approximately 18 kg, and injuries from carrying and lifting the blocks often result. Developments in material science have resulted in newer, lighter cementitious materials to replace the use of concrete block, particularly for the construction of retaining walls, stoppings, and seals.